TAS FET EUCLIDEAN GEOMETRY COURSE BOOKLET 3 Circle Geometry S A T by The TAS Maths Team Circle Geometry Geometry FET Course Booklets Set WWW.THEANSWER.CO. ZA "SUBTEND" . . . CIRCLE GEOMETRY P P O O Understand the word! The Language (Vocabulary) GROUP 1 AND Centre radius diameter centre A A A B Figure 2 S A T O A 180º Figure 3 ˆ a central AOB at the centre of the circle, and ˆ an inscribed APB at the circumference of the circle. Chords B To ensure that you grasp the meaning of the word 'subtend' : B Arcs (major & minor) A sector Copyright © The Answer Series P O move along the radii to meet at O and back ; then, minor arc AB move to meet at P on the circumference and back. Turn your book upside down and sideways. You need to recognise different views of these situations. Segments (major & minor) major segment A A Consider that subtend means support. Take each of the figures: chord AB Place your index fingers on A & B ; O Figure 4 major arc AB A Sectors O B or chord AB, subtends: In all the figures, arc AB (AB), Circumference chord B Central and Inscribed angles B O B Figure 1 C Diameter Radius 2 A P P minor segment Take note of whether the angles are acute, obtuse, right, straight or reflex. Redraw figures 1 to 4 leaving out the chord AB completely and observe the arc subtending the central and inscribed angles in each case. B B 1 GROUP GROUP 3 Cyclic Quadrilaterals Tangents A cyclic quadrilateral is a quadrilateral which has all 4 vertices on the circumference of a circle. Special lines D 4 A tangent is a line which touches a circle at a point. S A T Points A, B, C and D are concyclic, i.e. they lie on the same circle. O A tangent point of contact C B Note: Quadrilateral AOCB is not a cyclic quadrilateral because point O is not on the circumference ! (A, O, C and B are not concyclic) A secant is a line which cuts a circle (in two points). We name quadrilaterals by going around, either way, using consecutive vertices, i.e. ABCD or ADCB, not ADBC. secant Exterior angles of polygons The exterior angle of any polygon is an angle which is formed between one side of the polygon and another side produced. e.g. A triangle e.g. A quadrilateral / cyclic quadrilateral A E D B A C D B NB : It is assumed that the tangent is perpendicular C ˆ is an exterior ø of ΔABC. ACD [NB : BCD is a straight line ! ] Copyright © The Answer Series to the radius (or diameter) ˆ is an exterior ø of c.q. ABCD. ADE at the point of contact. [NB : CDE is a straight line ! ] 2 SUMMARY OF CIRCLE GEOMETRY THEOREMS I II III x The 'Centre' group The ' No Centre ' group The ' Cyclic Quad.' group 2x 2x x S A T x Equal chords ! x There are ' 3 ways to prove that a quad. is a cyclic quad '. y x + y = 180º IV The 'Tangent' group Equal tangents ! There are ' 2 ways to prove that a line is a tangent to a ?'. Copyright © The Answer Series 3 Equal radii ! GROUPING OF CIRCLE GEOMETRY THEOREMS The grey arrows indicate how various theorems are used to prove subsequent ones I II III x The 'Centre' group The ' No Centre ' group The ' Cyclic Quad.' group 2x 2x x S A T Equal chords ! x Equal chords subtend equal angles and, vice versa, equal angles are subtended by equal chords. x There are ' 3 ways to prove that a quad. is a cyclic quad '. y x + y = 180º IV The 'Tangent' group Equal tangents ! There are ' 2 ways to prove that a line is a tangent to a ?'. Copyright © The Answer Series Equal radii ! 4 2 GEOMETRY The angle which an arc of a circle subtends at the centre is double the angle it subtends at any point on the circumference. Given : ˆ at the centre and APB ˆ at the circumference. O, arc AB subtending AOB P ˆ = 2APB ˆ AOB To prove : Construction : Join PO and produce it to Q. Proof : Proofs required for Pˆ 1 = Let x Then  = x . . . s opp = radii Oˆ 1 = 2x . . . exterior of DAOP Similarly, if Pˆ 2 = y, then Oˆ 2 = 2y exam purposes 12 x y O 1 2 x y A Q ˆ = 2x + 2y = 2(x + y) = 2 APB ˆ AOB S A T 3 B The opposite angles of a cyclic quadrilateral are supplementary. A x Given : O and cyclic quadrilateral ABCD To prove : Circle Geometry Theorems 1 Proof : Construction : Join OA and OB In Ds OPA & OPB . . . given (3) OP is common DOAP º DOBP . . . RHS AP = PB, i.e. OP bisects chord AB A 1 2 P D 1 B . . . at centre = 2 at circumference = 360º – 2x . . . s about point O C 4 B quadrilateral = 360º The angle between a tangent to a circle and a chord drawn from the point of contact is equal to the angle subtended by the chord in the alternate segment. ˆ , Given : Tangent AC and chord AB subtending APB with C & P on opposite sides of AB. ˆ = APB ˆ , A 1 To prove : Construction : Draw diameter AD and join DP. Proof : We have ˆ = 90º DAC . . . diameter ⊥ tangent & ˆ = 90º DPA . . . in semi- ∴ Aˆ 1 + Aˆ 2 = 90º See Gr 11 Maths 3 in 1 Study Guide and But ˆ ˆ A 2 = P1 5 D P B 1 2 2 1 x A Pˆ 1 + Pˆ 2 = 90º . . . s in same seg. ˆ ∴ Aˆ 1 = Pˆ 2 ( = APB) Copyright © The Answer Series O x Ĉ = 1 (360º – 2x) = 180º – x . . . at centre = 2 2 at circumference  + Ĉ = 180º s sum of the of a ... & B̂ + D̂ = 180º O . . . radii 2 ˆ = 2x O 1 Oˆ 2 To prove : AP = PB (2) Pˆ 1 = Pˆ 2 (= 90º)  = Let Then Given : O with OP ⊥ AB (1) OA = OB B̂ + D̂ = 180º Construction : Join BO and DO. The line segment drawn from the centre of a circle, perpendicular to a chord, bisects the chord. Proof :  + Ĉ = 180º & Remember : Always give REASONS for each statement ! C PROVING THEOREMS S A T x y y x 2x Copyright © The Answer Series 6 x y 2y x y 2x 2y 2x 2y y x ? THEOREM PROOFS: A Visual presentation The Situation Examinable Theorem Statements The LOGIC . . . Construction radii 1 centre chord S A T Examin 2 radius produced inscribed ∠ central ∠ y y x arc x 2x 2y equal base ∠ s radii 3 xy x y centre exterior ∠ s of Δs y Circle Theorem Proofs The line drawn from the centre of a circle, perpendicular to a chord, bisects the chord. x x opposite ∠ s in a cyclic quadrilateral 2y y 2x ∠ at centre = 2 % ∠ at circumference & ∠ s about a point = 360º. Note: The bolded words in the statements are the approved 'reasons' to use. Copyright © The Answer Series 7 The angle which an arc of a circle subtends at the centre is double the angle it subtends at any point on the circumference. The opposite angles of a cyclic quadrilateral are supplementary. ? THEOREM PROOFS: A Visual presentation, continued The Situation Method 1: 4 inscribed ∠ diameter & join . . . chord x Method 2: radii x Theorem Statement The angle between a tangent to a circle and a chord drawn from the point of contact is equal to the angle subtended by the chord in the alternate segment. S A T 90° – x x 90° – x x x ∠ in the same segment 90° – x x x Tan Chord theorem x 2x 2x x x s ∠ at centre = 2 % ∠ at circumference sum of ∠ of Δ Copyright © The Answer Series 8 90° – x base ∠ s of isosceles Δ radius ⊥ tangent This theorem is known as the x s diameter ⊥ tangent & ∠ in semi-? tangent Circle Theorem Proofs The LOGIC . . . Construction Examinable FURTHER ? THEOREM PROOFS: A Visual presentation The Situation 5 ? S A T 180º diameter Central angle is a straight angle 6 The LOGIC . . . x x (at the circumference of the circle) arc 7 2x 2x ∠ subtended by an arc (or chord) (at the centre of the circle) ∠ s subtended by an arc (or chord) (at the circumference) x cyclic quadrilateral x ∠ s subtended by the same arc are equal. The exterior ∠ of a cyclic quadrilateral exterior angle y y Adjacent ∠ s : Copyright © The Answer Series The ∠ in a semi-? is a right ∠. Inscribed angle is a right angle Construction: radii ∠ s subtended by an arc Circle Theorem Proofs Theorem Statements The LOGIC . . . 9 x x + y = 180º = the interior opposite ∠. FURTHER ? THEOREM PROOFS: A Visual presentation, continued . . . The Situation The LOGIC . . . Construction Method 1: radii 8 S A T radii ⊥ tangents Method 2: Theorem Statement congruent Δs chord and inscribed ∠ x x x x Circle Theorem Proofs x x tan-chord theorem 4 Proofs 5 to 8 sides opposite equal base ∠ s in Δ are not examinable, but, the LOGIC is crucial when studying geometry. Copyright © The Answer Series x 10 Tangents to a circle from a common point are equal. Again, the bolded words are the ‘approved reasons' to use. EXAMPLE 1 PROBLEM-SOLVING: An Active Approach This is an excellent example testing so many ?-geometry theorems (excluding tangents). ACT ! A Be Active . . . Mark all the information on the drawing : equal or parallel sides equal øs right øs K Use all the Clues . . . intersecting lines adj. suppl. & vert. opp. øs ; øs about a point. ˆ (c) M 4 ˆ +N ˆ (d) N 1 2 parallel lines alternate, corresponding & co-interior øs ˆ (e) M 1 (f) Prove that KG = GM equal chords equal øs 90º angle the Theorem of Pythagoras, or . . . sum of the interior øs of a quadrilateral = 360º ; types & properties of all quadrilaterals F 76º K 1 O 2 1 3 2 H G 4 ˆ ˆ = 1O (c) M 4 1 12 N 2 ˆ +N ˆ = 180º - 76º (d) N 1 2 = 104º . . . opp. øs of c.q. are suppl. ˆ = 90º (e) KML . . . ø in semi-? ˆ = 38º in 4.3 ˆ &M ˆ â M 1 = 180º - (90º + 38º) . . . str. NMP 4 = 52º Previous geometry Revise previous geometry under the headings angles lines triangles quadrilaterals (f) ˆ = corresponding KML ˆ = 90º G 3 . . . ON || LM i.e. line from centre, OG ⊥ chord KM Circle geometry â KG = GM Recall and apply the facts by group. 11 L 1 2 3 2 1 = 38º . . . ø at centre = 2 % ø at circumference Recall the Theory systematically . . . Copyright © The Answer Series M N ˆ = corr. Lˆ . . . ON || LM (b) O 1 1 = 76º isosceles Δs ; congruency, similarity, theorem of Pythagoras s 1 2 (a) Lˆ 1 ( = Fˆ ) = 76º . . . øS in same segment Δs sum of the interior øs = 180º ; exterior ø of Δ ; equilateral Δs ; 3 2 1 Answers There are 3 theorems in ? geometry which involve right angles. diameter ø in semi-? = 90º ; diameter ⊥ tangent P 1 2 4 ˆ (b) O 1 L H 1 3 2 G (a) Lˆ 1 equal radii or tangents (in triangles) equal base øs O 2 1 S A T Calculate, giving reasons, the sizes of : The information provides clues to facts : T 76º ON || LM and F̂ = 76º. radii diameter tangents C F O is the centre of the circle and diameter KL is produced to meet NM produced at P. M P EXAMPLE 2 3 ways to prove that a quadrilateral is cyclic (a) Complete the following by writing the appropriate missing word. If a chord of a circle subtends a right angle on the circumference, then this chord is a . . . . . . B (b) A, B, C and D are points on the circle. 1 1 1 If M̂ = N̂, prove that AC is a diameter of the circle. Answers z1 y2 . . . given ˆ = C ˆ = y . . . vert. opp. øs & C 1 2 Prove that a pair of opposite angles is supplementary. N 1 x+y Then : Bˆ 1 = x + y . . . ext. ø of ΔBMC ˆ = x+y & D 1 . . . ext. ø of ΔDCN ˆ = 180º But, Bˆ 1 + D 1 â 2(x + y) = 180º â x + y = 90º ˆ ˆ = 90º i.e. B = D x B A  + Ĉ = 180º or B̂ + D̂ = 180º D 1 . . . opp. ø of c.q. 7 Either prove : 1̂ = 2̂ 4 8 3 6 1 y 2 C or 3̂ = 4̂ or 5̂ = 6̂ or 7̂ = 8̂ 2 ways to prove that a line is a tangent to a ? 2 x N We use the 2 converse theorem statements to prove that a line is a tangent to a ? if . . . converse theorem 1 â AC is a diameter . . . subtends a right ø ACT! . . . it is perpendicular to a radius at a point where the radius meets the circumference. O Prove that OP ⊥ AB; then AB will be a tangent A P B converse theorem A: Be Active . . . when drawn through the end point of a chord, it makes with the chord an angle equal to an angle A in the alternate segment. C: Use all your Clues T: Apply the Theory systematically, recalling each group, and fact, one at a time. Copyright © The Answer Series 5 2 C D Either prove : A Prove that an exterior angle of a quadrilateral is equal to the interior opposite angle. M Either prove : x1 = x2 or y1 = y2 or z1 = z2 or p1 = p2 y1 x+y s 2 z2 p2 B converse theorem (a) . . . diameter. p1 x 2 y1 converse theorem 2 S A T D x1 Prove that one side subtends equal angles at the two other points (on the same side). C A AB produced and DC produced meet at M. 1 converse theorem 2 2 BC produced and AD produced meet at N. (b) Let M̂ = N̂ = x We use the 3 converse statements for cyclic quadrilaterals. M 12 y x B Prove that x = y; then AB will be a tangent Note : It is a good idea to draw a faint circle (as shown) to see this converse theorem clearly. EXAMPLE 3 (National November 2017 P2, Q10) (b) ˆ = 90º . . . tangent ⊥ radius OWS ˆ ˆ â OVN = OWS (= 90º) (i) In the diagram, W is a point on the circle with centre O. â MN || TS . . . corresp. øs equal V is a point on OW. (ii) Chord MN is drawn such that MV = VN. The tangent at W meets OM produced at T and ON produced at S. O T M 1 2 1 (a) Give a reason why OV ⊥ MN. (b) Prove that: (i) MN || TS (ii) TMNS is a cyclic quadrilateral V W 2 O S A T M T 1 1 2 1 2 Shade the quadrilateral TMNS V 2 N W S (iii) OS . MN = 2ON . WS (a Grade 12 question) Answers The most 'basic' way to prove lines || is: alt. or corresp. øs equal or co-int. øs suppl. 1 2 N S There are 3 ways to prove that a quadrilateral is a cyclic quadrilateral – choose 1: (a) Line (OV) from centre to midpoint of chord (MN) y x In this case, the midpoint of the chord is given, and we can conclude that OV ⊥ MN because of that. Prove that: x + y = 180º Prove that: Ext. ø = int. opp. ø s ˆ = N ˆ M 1 1 . . . ø opposite equal radii Note: Analyse the information and the diagram. = Ŝ So far, we have used and applied a 'centre' theorem, in (a). Another clue is the 'tangent' at W. Think about tangent facts . . . . . . . corresp. øs ; MN || TS â TMNS is a cyclic quadrilateral Copyright © The Answer Series 13 Prove that: A side subtends equal øs at 2 other vertices We chose and proved that the exterior ø of quadrilateral TMNS = the interior opposite ø converse ext. ø . . . of cyclic quad. (iii) EXAMPLE 4 This question looks like ratio and proportion. Mark the sides on the diagram. Don't be put off by this drawing ! Direct your focus to one situation at a time ☺ The sides appear to involve ΔOWS, which has VN || WS, (even though MN = 2VN) . . . Maybe apply the proportion theorem in this Δ? A But, the sides in the question involve the horizontal sides WS and VN. 1 2 So, proportion theorem is excluded. We will use similar Δs ! S A T X In Δs OVN and OWS ˆ is common O 2 ˆ = OWS ˆ OVN 1 2 1 â ΔOVN ||| ΔOWS . . . øøø B 2 2 Q Y 3 P . . . corresp. øs ; MN || TS 1 1 2 T Make statements, with reasons, Let's 'arrange' the sides to suit the question. OS â = WS ⎛⎜ = OW ⎞⎟ ON VN ⎝ OV ⎠ . . . equiangular Δ 1. In ?XPBA : about P̂1 and B̂1 s ˆ 2. In ?ABYQ : about B̂1 and AQY â OS . VN = ON . WS But VN = 1 MN 2 ˆ 3. In quadrilateral APTQ : about P̂1 and AQT . . . V midpoint MN â OS . 1 MN = ON . WS 4. What can you conclude about quadrilateral APTQ? 2 Mark the øs on the drawing as you proceed. % 2)â OS . MN = 2ON . WS Answers O M T 2 1 2 1 V 1 W 2 . . . arc XA subtends øs in same segment ˆ 2. In c.q. ABYQ : B̂1 = AQY . . . exterior ø of cyclic quad. ˆ 3. In quad. APTQ : P̂1 = AQT N 4. ∴ APTQ is a cyclic quad. S Copyright © The Answer Series 1. In c.q. XPBA : P̂1 = B̂1 14 . . . both = Bˆ1 above . . . converse of exterior ø of cyclic quad. EXAMPLE 5 EXAMPLE 6 P Prove that PA is a tangent to M. 8 AB is a tangent to the circle at B and BD is a chord. T AD cuts the circle in E. 12 C is a point on BD so that ABCE is a cyclic quadrilateral. M 3 A 4 AC, BE and CE are joined. B Q A 1 2 Answers In right-d ∆MAQ : AM = 5 units . . . 3 :4 :5 ∆ ; Pythag. . . . TM = AM = radii TM = 5 units PM = 13 units P 8 12 S A T B A i.e. PM 2 = PA2 + AM 2 1 2 2 M 3 ∆PAM is a 5 : 12 : 13 ∆ ! ˆ = 90º PAM T 1 4 Q 3 E 3 21 C D B Prove that : (a) AB = AC . . . converse of Pythag. (b) AC is a tangent to circle ECD at point C. PA is a tangent to M . . . converse tan chord theorem Answers B̂1 = x (a) Let then D̂ = x . . . tan chord theorem and Ĉ2 = x . . . s in same segment & See Gr 11 Maths 3 in 1 Study Guide A 1 2 y Let B̂2 = y then  2 = y Ĉ3 = D̂ +  2 . . . s in same segment B ∴ AB = AC Module 9b: Circle Geometry Notes Exercises Full Solutions (b) Ĉ2 = D̂ . . . B̂1 = x and . . . converse tan chord thm ∴ AC is a tangent to ECD at C. Copyright The Answer Series 15 B̂2 = y . . . sides opp = s . . . both = x in (a) 1 x 2 y x 3 21 C . . . ext. of D = x+y ˆ = x+y & ABC 1 2 3 E x D ACCEPTABLE REASONS: CIRCLE GEOMETRY THEOREM STATEMENT O O O O O x O 2x ACCEPTABLE REASON(S) The tangent to a circle is perpendicular to the radius/diameter of the circle at the point of contact. tan ⊥ radius tan ⊥ diameter If a line is drawn perpendicular to a radius/diameter at the point where the radius/diameter meets the circle, then the line is a tangent to the circle. line ⊥ radius OR The line drawn from the centre of a circle to the midpoint of a chord is perpendicular to the chord. The line drawn from the centre of a circle perpendicular to a chord bisects the chord. The perpendicular bisector of a chord passes through the centre of the circle. The angle subtended by an arc at the centre of a circle is double the size of the angle subtended by the same arc at the circle (on the same side of the chord as the centre) O The angle subtended by the diameter at the circumference of the circle is 90º. O If the angle subtended by a chord at the circumference of the circle is 90°, then the chord is a diameter. Copyright © The Answer Series THEOREM STATEMENT x x y y x Angles subtended by a chord of the circle, on the same side of the chord, are equal øs in the same seg If a line segment joining two points subtends equal angles at two points on the same side of the line segment, then the four points are concyclic. line subtends equal øs S A T converse tan ⊥ radius OR x converse tan ⊥ diameter ACCEPTABLE REASON(S) OR converse øs in the same seg (This can be used to prove that the four points are concyclic). line from centre to midpt of chord x x Equal chords subtend equal angles at the circumference of the circle. equal chords; equal øs Equal chords subtend equal angles at the centre of the circle. equal chords; equal øs line from centre ⊥ to chord x xO perp bisector of chord x øat centre = 2 % ø at circumference x A x B x Equal chords in equal circles subtend equal angles at the circumference of the circles. Equal chords in equal circles subtend equal angles at the centre of the circles. (A and B indicate the centres of the circles) øs in semi circle OR diameter subtends right angle OR ø in ½ ? x chord subtends 90º OR y The opposite angles of a cyclic quadrilateral are supplementary (i.e. x and y are supplementary) 180º – x If the opposite angles of a quadrilateral are supplementary then the quadrilateral is cyclic. x converse øs in semi circle 16 equal circles; equal chords; equal øs equal circles; equal chords; equal øs opp øs of cyclic quad opp øs quad sup OR converse opp øs of cyclic quad THEOREM STATEMENT The exterior angle of a cyclic quadrilateral is equal to the interior opposite angle. x x If the exterior angle of a quadrilateral is equal to the interior opposite angle of the quadrilateral, then the quadrilateral is cyclic. x x B Two tangents drawn to a circle from the same point outside the circle are equal in length (AB = AC) A C x x y x y a b y ACCEPTABLE REASON(S) ext ø of cyclic quad ext ø = int opp ø OR converse ext ø of cyclic quad S A T Tans from common pt OR Tans from same pt The angle between the tangent to a circle and the chord drawn from the point of contact is equal to the angle in the alternate segment. tan chord theorem If a line is drawn through the endpoint of a chord, making with the chord an angle equal to an angle in the alternate segment, then the line is a tangent to the circle. converse tan chord theorem (If x = b or if y = a then the line is a tangent to the circle) Copyright © The Answer Series NOTES OR ø between line and chord 17 PLEASE NOTE S A T These Geometry materials (Booklets 1 to 4) were created and produced by The Answer Series Educational Publishers (Pty) (Ltd) to support the teaching and learning of Geometry in high schools in South Africa. They are freely available to anyone who wishes to use them. This material may not be sold (via any channel) or used for profit-making of any kind. GRADES 8 - 12 ALL MAJOR SUBJECTS IN ENGLISH & AFRIKAANS S A T WWW.THEANSWER.CO. ZA